Thermal relay



Patented Mar. 6, 1945 THERMAL RELAY Harry E. Ashworth, Penn Township,Allegheny County, and Selden B. Aylsworth, Edgewood, Pa... assignors toThe Union Switch & Signal Company, Swissvale, Pa., a corporation ofPennsylvania Application April 16, 1943, Serial No. 483,260

8 Claims.

Our invention relates to thermal relays, and particularly to thermalrelays which employ a bimetallic strip heated by a heater element foractuating contacts at the expiration of a predetermined time intervalafter the application of power to the relay.

The principal object of our invention is to improve the operatingcharacteristics of a relay of the type described by providing morenearly uniform performance under varying conditions of ambienttemperature.

Relays embodying our invention are an improvement upon the relaydescribed and claimed in Letters Patent of the United States No.2,114,895, granted to Harry E. Ashworth on April 19, 1938.

According to our invention, the relay comprises a first bimetallic stripsupported at one end through the medium of an oppositely disposedbimetallic strip which compensates insofar as possible for the effectsof ambient temperature on said first strip. A metal bracket is securedto said first strip adjacent to its free end and is provided with anoffset portion carrying a heating element. The offset portion is sodisposed that the heat generated by said heating element will betransferred to said first strip both by conduction through said bracketand by radiation across the intervening space to cause said first stripto deflect, and that as said first strip deflects said heater elementwill move closer to said first strip and will thus increase the heattransfer in a manner to compensate for any tendency of the strip todecrease its rate of bending as the amount of deflection increases tothereby provide a nearly linear relationship between distance moved andtime required to move such distance. This compensation is effectivethroughout the range of ambient temperatures to which thermal relays foruse in railway signaling systems are generally subjected. To furthercompensate for the efiects of changes in ambient temperature on thecharacteristics of the relay the heating element is covered by meanswhich regulates the rate of heat transfer from the heating element tosaid first strip to the proper value to obtain a nearly uniformoperation which is independent of variation in ambient temperature.

Other objects and characteristic features of our invention will becomeapparent as the description proceeds.

We shall describe one form of relay embodying our invention, and shallthen point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a front elevational view showingone form of relay embodying our invention, the parts being in theirnormal positions. Fig. 2 is a view similar to Fig. 1 but showing theparts in their operated positions. Figs. 3 and 4 are side elevationaland bottom views of the thermal element TE of the relay shown in Fig. 1.Fig. 5 is an enlarged side view of a portion of the thermal element TEof the relay shown in the preceding views. Fig. 6 is a sectional viewtaken on the line VI-VI of Fig. 5. Fig. '7 is a sectional view taken onthe line VII-VII of Fig. 5.

Similar reference characters refer to similar parts in each of theseveral views.

Referring first to Figs. 1, 3 and 4-, the relay comprises a top plate Iof suitable insulating material, such as Bakelite, which top platesupports all of the operating parts of the relay. Secured to the topplate I in spaced relation are three terminal posts 2, 3 and 4 whichsupport respectively a fixed front contact 5, a thermal element TE, anda fixed back contact 6. The front contact 5 and back contact 6 are ofwell known constructions which are in widespread commercial use, andinasmuch as they 'form no part of our present invention it is deemedunnecessary to describe them in detail herein. It should be pointed out,however, that these contacts are each provided with suitable means foradjusting their positions relative to the thermal element TE to enablethe desired time calibration to be obtained.

The thermal element TE comprises a main bi metallic strip secured at itsupper end to an oppositely disposed auxiliary bimetallic strip 8, whichlatter, in turn, is rigidly fastened to a rigid strip 9 secured within atransverse slot It! provided in the head of the terminal post 3. Themain strip 1 is provided at its lower end with an insulating block IIcarrying a low resistance contact member l2 for cooperation with theback contact 6, and with a metal bracket 13 carrying a low resistancecontact member I4 for cooperation with the front contact 5. Contactmember 22 is provided with a flexible connector 15 which is adapted tobe secured to a suitable terminal post (not shown) mounted on the topplate I. Contact member I4 is electrically connected to the strip Ithrough the bracket l3 so that electrical connection with this contactmember can be made at terminal post 3.

The bracket I3 is provided with an offset portion I312 which carries aheating element 16. The heating element is insulated from the offsetportion l3a by suitable insulating means here shown as a layer 2| ofasbestos paper, and has its terminals I611 and "lb connected to flexibleconnections I! and I8 by means of rivets l9 and 20 mounted in theinsulating block ll adjacent the opposite sides thereof. The flexibleconnectors I1 and I8 are intended to be connected at their free ends tosuitable terminal posts (not shown) mounted on the top plate I.

When the parts are at, ambient temperature, the contact member l2engages the contact member 6 to close the contact 12-6, and the cont-actmember i4 is spaced from the contact member so that the contact l4-5 isopen. When heating'element I6 is energized the heat is transferred tostrip 1 by conduction through the bracket I3 and also by radiation, andthis heat causes the strip 1 to deflect in the manner shown in Fig. 2,thereby opening contact l2-6 and closingcontact M5. The opening ofcontact |26 will occur at the expiration of a very short time intervalafter the heating element 16 becomes energized, but due to thecomparatively slow rate of heat transfer from the heating element to thestrip 1, contact l4-5 will not become closed until the, expiration of arelatively long time interval after heating element is becomesenergized.

Thermal relays of the type dscribed are frequently employed in railwaysignaling systems to provide a predetermined time interval between thefunctioning of related circuits of the system, and when so used it isessential that the time interval be as nearly as possible independent ofvariations in ambient temperature. Where the variations in ambienttemperature are relatively small the necessary compensation can beobtained by means of the auxiliary bimetallic strip 8, which, beingoppositely disposed with respect to the main strip I, deflects in theopposite direction to the strip I in response to changes in ambienttemperature, and which is of such length that the deflection of thisstrip for small variations in ambient temperature will just offset thedeflection of the main strip 1 so that the contact end of the strip 1will remain in a substantially fixed position with respect to theassociated fixed front and back contact members 5 and 6. We have found,however, that at the extremes of ambient temperature at which relays ofthe type described must at times operate, the compensation that can beobtained by the auxiliary strip 8 is not complete and that as a result,if no other compensation is provided, there will be a considerablevariation at these extremes between the time interval for any particularsetting and voltage and that obtained for the same setting and-voltageat normal ambient temperatures. That is to say, any particular relaywill vary considerably from its desired time interval with variations inambient temperature in spite of the thermal compensation provided by theauxiliary strip 8.

This variation is undesirable, and appears to be due at least in part tothe fact that the rate of bending of a bimetallic strip, when actuatedby heat, decreases as the amount of deflection increases, the actionbeing such that the strip appears to increase in stiffness withincreases in deflection. This results in a non-linear relation betweenthe distance moved by the free end of the strip and the time requiredfor the strip to move such distance, and in accordancewith our presentinvention we overcome this nonlinear relationship by so disposing theoffset portion of the bracket I3 that as the strip 1 deflects due to theheat generated in the heating element IS the heating element will movecloser to the strip 1 as shown in Fig. 2, and will thus increase therate of heat transfer in a manner to compensate for the tendency of thestrip to decrease its rate of bending with increases in deflection.Furthermore, we also provide means for conserving the heat generated bythe heating element and for'causing it to pass to the bi- JBtaHlC strip1 at the correct rate to maintain a nearly uniform rate of bendingregardless of the ambient temperature. This latter means comprises athin piece 24 of mica disposed on the side of the heating element l6farthest from the bimetallic strip, a turn and a half of asbestos paper22 surrounding the element and said sheet of mica and lapped on the sameside of the element as the mica, and a copper band 23 which surroundsthe asbestos and serves as a means to clamp both the mica and theasbestos in place. These coverings act to prevent excessive radiation ofheat from the heating element at the side farthest from the bimetallicelement, insofar as possible, and at the same time store heat in justsuificient quantity to cause the relay to operate as desired. The wholeefiect is to cause the relay to operate at wide extremes in temperaturewith an exceedingly small percentage of variation in time. In actualtests, the variation in time with the covering is about one-half thatobtained without the coverings.

Although we have herein shown and described only one form of relayembodying our invention. it is understood that various changes andmodifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In a thermal relay, in combination, a thermal element comprising abimetallic strip supported at one end and provided at the other end withmeans for operating contacts at the expiration of a time interval inresponse to heating of said strip, and means for heating said stripsecured to said strip by means which causes the heat supplied to saidstrip to increase as the deflection of said strip resulting from saidheat increases.

2. In a thermal relay, in combination, a thermal element including afirst bimetallic strip supported at one end by an oppositely disposedbimetallic strip which at least partly compensates for changes inambient temperature, and a heating element for heating said first stripmounted on a bracket secured to the free end of said strip, said bracketbeing so constructed that as said strip deflects due to the heatsupplied thereto by said heating element the distance between saidheating element and said strip will decrease to further compensate forchanges in ambient temperature.

3. In a thermal relay, in combination, a thermal element including abimetallic strip supported at one end, a bracket secured to said stripand provided with an ofisetportion which moves closer to said strip inresponse to deflection of said strip due to heating of the strip, athermal element mounted on but insulated from said offset portion, andmeans for conserving the heat generated by said heating element and forcausing it to pass to the bimetallic strip at the correct rate tomaintain a nearly uniform rate of bending regardless of variations inambient temperature, said means comprising a piece of mica disposed atthe side of said heating element farthest from said strip, a turn and ahalf of asbestos paper surrounding said element and said sheet andlapped on the outer side of said mica, and a metal band surrounding theasbestos and serving as a means to clamp both said mica and said paperin place.

4. In a thermal relay, in combination, a thermal element including afirst bimetallic strip supported at one end by an oppositely disposedbimetallic strip which at least partly compensates for changes inambient temperature, a heating element for heating said first stripmounted on a bracket secured to the free end of said strip, said bracketbeing so constructed that sheet and lapped on the outer side of saidmica,

and a metal band surrounding the asbestos and serving as a means toclamp both said mica and said paper in place.

5. In a thermal relay, in combination, a thermal element including abimetallic strip supported at one end, a bracket secured to said stripand provided with an offset portion which moves closer to said strip inresponse to deflection of said strip due to heating of the strip, athermal element mounted on. but insulated from said offset portion, andmeans for conserving the heat generated by said heating element and forcausing it to pass to the bimetallic strip at the correct rate tomaintain a nearly uniform rate of bending regardless of variations inambient temperature, said means comprising an insulating coveringsurrounding said heating element.

6. In a thermal relay, in combination, a thermal element including abimetallic strip supported at one end, a bracket secured to said stripand provided with an offset portion which moves closer to said strip inresponse to deflection of said strip due to heating of the strip, athermal element mounted on but insulated from said offset portion, andmeans for conserving the heat generated by said heating element and forcausing it to pass to the bimetallic strip at the correct rate tomaintain a nearly uniform rate of bending regardless of variations inambient temperature, said means comprising an asymmetrical insulatingcovering surrounding said heating element.

'7. In a thermal relay, in combination, a thermal element including a,bimetallic strip supported at one end, a, bracket secured to said stripand provided with an offset portion which moves closer to said strip inresponse to deflection of said strip due to heating of the strip, athermal element mounted on but insulated from said offset portion, andmeans for conserving the heat generated by said heating element and forcausing it to pass to the bimetallic strip at the correct rate tomaintain a nearly uniform rate of bending regardless of variations inambient temperature, said means comprising an asymmetrical insulatingcovering surrounding said heating element and disposed with the maximuminsulation at the side opposite said bimetal strip.

8. Ina thermal relay, in combination, a thermal element including abimetallic strip supported at one end, a bracket secured to said stripand provided with an offset portion which moves closer to said strip inresponse to deflection of said strip due to heating of the strip, athermal element mounted on but insulated from said oilset portion, andheat insulating means associated with said heating element forconserving the heat generated by said heating element and for causing itto pass to said bimetallic strip at the correct rate to maintain anearly uniform rate of bending regardless of variations in ambienttemperature.

, HARRY E. ASHWORTH.

SELDEN B. AYLSWORTH.

